PDCD1 and helminthiasis: Furthermore, expression levels of programmed cell death-1 (PD-1), lymphocyte activation gene 3 (LAG-3), killer cell lectin-like receptor subfamily G member 1 (KLRG-1), CD39, T-cell immunoglobulin and mucin-domain containing-3 (Tim-3), and IL-10 have been associated with an exhausted phenotype, and some of these markers have been associated with adaptive immunity during helminth infections (Layland et al., 2010; Zander and Butler, 2013; Wherry and Kurachi, 2015; Taylor et al., 2017; Lima et al., 2018).